Solid-state laser

ABSTRACT

A solid-state laser is proposed with an emission wavelength of greater than 1.4 μm to eliminate the risk of damage to human skin and eyes. The proposed laser comprises: an active medium alloyed with Er 3+   ions; an optical pumping source separated from the active medium by an additional filtering layer in the form of optical elements, and/or coating applied thereto, and/or a liquid medium, which intercept and eliminate ultraviolet radiation and are transparent to light in the excitation wavelength of the active medium. The cumulative internal transmission density of the filtering layer along the path of a given beam in the laser from the pumping source to the active medium, including the path in the casing of the pumping source, must be greater than 2 in the wavelength range below 320 nm and not more than 0.1 in the excitation spectrum of the active medium with wavelengths greater than 360 nm. Filtration in the laser is accomplished with the aid of, for example, a 0.3-1.5 μm thick film of cerium oxides applied to the surface of the transparent material of the laser components (CeO 2  accounting for not less than two thirds of the oxides in question) and/or using a liquid medium containing cerium compounds, e.g. based on an aqueous solution of the salts CeCl 3  and/or CeBr 3  with a sum molar concentration of Ce 3+   ions of at least 0.4 mol/l.

BACKGROUND OF THE INVENTION

The present invention relates to the field of quantum electronics, andmore particularly to lasers emitting at wavelengths of above 1.4 μm.

DESCRIPTION OF THE PRIOR ART

Known are lasers using yttrium aluminum garnet (YAG) and yttriumaluminate (YAlO₃) crystals doped with trivalent erbium ions (Er³⁺), inwhich, to reduce the threshold lasing energy at transitions between ⁴I_(11/2) and ⁴ I_(13/2) levels at a wavelength of about 3 μm,wavelength-selective filters isolating the active element from thepumping lamps and absorbing the pumping light with wavelengths below 645to 545 nm are used. These filters interposed between the active mediumand the pumping light source comprising one or several lamps are made inthe form of filtering additives either to the liquid medium surroundingthe pumping light source lamp(s) and/or the active element ( J. "OpticsLetters", vol. 13, No. 11, 1988, J. Frauchiger et al.: "Laser Propertiesof Selectively Excited YAlO₃ :Er",p.p.964-966), or to the material ofthe optical component isolating the active medium from the pumping lightsource (IEEE J. Quant. Electr., vol. QE-28, No. 11, 1992, J. Breguet etal.: "Comparison of Threshold Energy of Selectively Excited YAlO₃ :Erand YAG:Er Lasers", p.p. 2563-2566).

The use of such lasers at the propagation of their radiation throughhumid atmosphere over large distances is a matter of some difficultybecause a considerable absorption in atmospheric water vapors isobserved at their lasing wavelength of about 3 μm.

It is also known that lasing has been achieved in YAlO₃ crystals dopedwith Er³ + ions in four-level operation at transitions between ⁴ S_(3/2)and ⁴ I_(9/2) levels at emission wavelengths between 1.6 and 1.8 μmwhich coincides with the high atmospheric transmission region ( J."Phys. D: Appl.Phys.", vol. 17, 1984, B.Dishler et al.: "Investigationof the laser materials YAlO₃ :Er and LiYF₄ :Ho", p.p. 1115-1124).

In such solid-state lasers using an active medium based on erbium-dopedYAG and YAlO₃ crystals the liquid medium isolating the pumping lightsource from the active medium comprises filters based on complex salts (for example, NaNO₃) and on various organic compounds ( IEEE J. Quant.Electr., vol. QE-23, No.2, 1987, M. Datwyler et al.: "New Wavelengths ofthe YAlO₃ :Er Laser", p.p. 158-159). These filters have transmissionbands in the spectrum of the said salts and organic compounds over theUV wavelength range, which cause the active medium degradation. They donot provide the desired useful life of lasers because of incomplete UVfiltering, due to which the lasers have a low efficiency and a highthreshold pumping energy the minimum value of which is 34 to 55 J.

In the U.S. Pat. No. 4,039,970, 1977, H01S 3/092 is described asolid-state laser with a light filter, which comprises an active mediumdoped with erbium ions and a pumping light source with an envelope madeof a pumping light-transmitting material, which is isolated from theactive medium by a gaseous and/or liquid medium and/or opticalcomponents intercepting and removing the pumping light short-wavelengthcomponent with wavelengths below 500 nm. Such a laser has an enhancedlasing threshold, 50 J, at the output mirror transmittance of 5%, butinsufficient lasing efficiency, 3 mJ, at a wavelength of 1.663 nm and apumping energy of 100 J.

In the EP 0,427,856 A1, H01S 3/092 document is described a solid-statelaser comprising short-wavelength radiation absorbing volume filterswith a small content (up to 1.5%) of rare-earth metals, including cerium(Ce). Such filters have a low optical density in the UV part of thespectrum. This causes the active medium degradation photo-induced by UVradiation resulting in the formation of color centers which absorb thepumping radiation over the Er³⁺ ion excitation range thus reducing thelasing efficiency and enhancing threshold pumping energy.

Known are lasers comprising protective coatings in the form ofalternately arranged layers of CeO₂, silicon oxides and aluminum oxide(Al₂ O₃) on the reflectors of not above 0.25 μm in thickness each, whichare designed for protection of the reflector silver surfaces from thecoolant (document SPIE vol. 609 Flashlamp Pump Laser Technology, 1988,Klaus D. Hachfeld: "The Engineering Art of Solid State Laser Pump CavityDesign", p.p. 55-77). These multilayer coatings have a high interferencetransmission selectivity at a strong dependence of reflection factorupon the light wavelength and incident angle. In case such coatings areused as filters, these properties cause a loss of the pumping lightincident onto the coating surface at wide angle and spectral ranges.Because of the small CeO₂ layer thickness they insufficiently absorb thepumping light UV component, which fact prevents them from being used inEr³⁺ lasers due to the active medium degradation. Moreover, all thatreduces the efficiency and increases threshold pumping energy of thelaser.

SUMMARY OF THE INVENTION

The basis of the invention is the task to devise a laser using Er³⁺-doped active media with lasing wavelengths of 1.6 to 1.8 μm and/or 2.7to 3.0 μm having enhanced efficiency and useful life and a reducedthreshold pumping energy. The set task is achieved by that in a laserusing a solid medium doped with trivalent erbium ions (Er³⁺) the pumpinglight source with an envelope of a transparent material is isolated fromthe active medium by an additional filtering layer in the form ofoptical components and/or coatings thereon and/or a liquid mediumintercepting and removing the radiation short-wavelength component,while being transparent over the active medium excitation range, so thatthe optical density of the filtering layer total over the full length ofany ray path from the pumping source to the active medium, including thepath in the pumping source envelope, should be at least 2 over thewavelength range below 320 nm and should not exceed 0.1 over the activemedium excitation spectral bands of wavelengths above 360 nm. The givendensity D(λ) of at least 2 at λ<320 nm is determined by the fact that ata narrow pumping pulse ofτ˜100 μs corresponding to the lifetime of themetastable state of the ⁴ S_(3/2) level of Er³⁺ ions and at the requiredpumping intensity the UV component is greater than the remaining part ofradiation by about 10 fold. In crystals and glasses, Er³⁺ ions areassociated with a great number of UV-sensitive color centers, includingthose based on Fe²⁺ ions, which are formed under the action of light atλ=300 to 320 nm. This determines the long-wavelength spectral regioncutoff with a high filter density at a wave-length of not below 320 nm.The filter density over the specified wavelength range should be suchthat intense UV radiation is attenuated by a factor of at least 100. Forthis purpose the filter transmission should not exceed 1%; that is, thefilter density over the filter reject range should be at least 2. Such afilter density value can be practically measured with availablespectrographic facilities during industrial application of the filter.

Therewithal, the Er³⁺ ion excitation lines having the highest absorptionfactor and making the greatest contribution to excitation are in the 355to 525 nm range, and at lasing from the ⁴ S_(3/2) level at thefour-level scheme provide about 90% of the total pumping. At thethree-level lasing scheme with Er³⁺ ion pumping over the up to about 1.1μm range the contribution of these lines reaches about 80%, while thecontribution of lines in the 556 to 1100 nm region is about 12%.Therefore, a reduction of the filter transmission at the transmissioncutoff should not cause the removal by absorption, reflection orconversion of more than 20% of the incident pumping light with awavelength of 360 nm, and the transmission should increase up to about95% as the wavelength increases. This satisfies the requirement for thefilter density to be not more than 0.1 at a wavelength of 360 nm andmakes it possible to utilize a maximum of the pumping light energy overthe range of the most effective excitation of erbium ions. The fullwidth of the transmission cutoff with the filter transmission changingfrom 0 to 80%, or the optical density from 2 to 0.1, should not exceed40 nm. This enhances the efficiency and reduces the threshold pumpingenergy of four- and three--level Er3⁺ lasers at wavelengths of 1.6 to1.8 μm and 2.7 to 3.0 μm, respectively.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic elevation of a preferred embodiment having acylindrical pumping light source isolated from the active medium byfilter coatings and a liquid medium.

FIG. 2 is a cross-sectional elevation of the invention shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The addition of the lengths of the light path sections from the pumpingsource to the active medium, with taking into account the spectraldependence of the material transmission factor on each section of thepath, is required for determining the light energy loss in all mediaand/or materials of all optical components through which the light rayspass. The reflector may itself consist of one or several monolithicoptical components of various shapes and may comprise optical componentsin the form of cylinders or flat plates of a transparent materialisolating and/or surrounding the light source and/or the active medium.For various rays with various wavelengths the optical components mayintroduce different losses depending on the actual optical path of raysin them and on the spectral dependence of the absorption factor, butshould not transmit radiation with wavelengths shorter than 320 nm,which causes the active medium degradation. The inclusion of the pumpinglight path inside the pumping source envelope into the total path lengthof any light ray from its source to the active medium is dictated bythat in determining the total optical density of the filter layer theportion of light already removed in the light source envelope should betaken into account in order to prevent an uncontrolled shift of thetotal filter transmission cutoff into the wavelength region above 360nm, in case the light source is changed or modernized, and to preventthe erbium ion excitation energy loss entailed with that; that is, inorder to obtain a filter density of not exceeding 0.1 at a wavelength of360 nm. The most effective is the application of a filter in the form ofa uniform-thickness layer. This provides uniform removal of the UVcomponent and uniform transmission of the pumping light for all rayspassing through any filter areas. Besides, at the uniformity of thefiltering dopant concentration, C, the uniformity of the filterthickness, d, allows to obtain a predetermined density, D(μ)=ε(μ)Cd,proportional to the filter extinction, ε(μ). In this case, the energyloss in the pumping light source envelope or in the coating can bereadily taken into account. The non-uniformity of the absorbing filterthickness requires accurate checking of the optical density, D(μ), and ahigh standard of technique of changing the filter extinction, ε(μ), ordopant concentration, C, depending on the filter thickness, d. Otherwisethat may cause blurring of the filter transmission cutoff (whereas therequirement for the filter transmission cutoff width to be 40 nm in theclaimed laser is rather stringent), which would result either inoverlapping of the Er³⁺ ion excitation bands or in transmission of thepumping light UV component. The solid active medium doped with trivalenterbium ions (Er³⁺) can be made in the form of either an integral(monolithic) or composite (assembled of cylinders, prisms, plates,discs, etc.) mono- or polycrystalline ceramic or glass active elements,active fibers or fiber bundles. The pumping light sources, includingdischarge lamps of any shape (flat, cylindrical, helical, coaxial, etc.)can be disposed outside the active medium (for example, near acylindrical or composite active element or active fiber bundle), cansurround the active medium (for example, when the active medium in theform of an element or a group thereof or a fiber bundle is mountedinside a helical or coaxial lamp), or can be disposed inside the activemedium (for example, when a cylindrical lamp is mounted inside a coaxialelement or a linear or helical fiber bundle).

The set task is also achieved by that used as a UV filter is a harddurable film of cerium compounds opaque in the short-wavelength lightrange and having appropriate filter coating transmission spectrumcharacteristics. For example, it can be a filter made in the form of a0.3 to 1.5 μm thick hard durable film of cerium oxides carrying at least2/3 of their stoichiometric composition as tetravalent cerium dioxide(CeO₂), which is deposited directly to the surface of the transparentmaterial of the pumping light source envelope and/or optical componentand/or active medium passed through by the pumping light. The use ofsuch a layer of cerium oxides in a moderate average pumping power laserof any type, uncooled or cooled with organic coolants or air, allows toprovide a minimum filter transmission cutoff width just within the 320to 360 nm range. Due to the increased broadening of lines, in case Ceoxides are used, Ce⁴⁺ ions with the absorption spectrum shifted into theUV region should be employed, which does not cause the absorption edgegeneral shift because of the absorption line broadening beyond 360 nm.The substitution of more than 1/3 of the content for trivalent ceriumoxide (Ce₂ O₃) makes the transmission cutoff more gently sloping withthe edge extending up to 400 nm. In this case, one of the mainrequirements for the filter density over the wavelength range of above360 nm is not satisfied. However, to dispose of other cerium oxidescompletely is difficult technologically. Therefore, taking intoconsideration the spectral requirements, the filter should carry atleast 2/3 of its composition as CeO₂, which provides both the requiredspectral characteristics and wide industrial applicability of thefilter. An advantage of the claimed laser is the greatly widenedallowable filtering layer thickness range, 0.3 to 1.5 μm, where thelower limit exceeds the layer thickness in the known devices. The upperlimit of the thickness range is restricted for both spectral reasons(because of a possible transmission cutoff shift into the wavelengthrange above 360 nm) and technological reasons (because of a reduceddurability of a thick film and an additional light loss over the entirerange due to the photo-induced recrystallization of the film).

The set task is also achieved by providing conditions for lightpropagation from the pumping light source to the active medium at aminimum light loss at the interface between the cerium oxide film andthe gaseous or liquid medium under the action of a high pulse pumpingslight power.

For this purpose a transition layer in the form of a film of aluminumtrioxide (Al₂ O₃) of 0.3 to 1.0 μm in thickness and/or of silicondioxide (SiO₂) of 0.5 to 1.0 μm in thickness is additionally depositedto the surface of the filter made in the form of a film of cerium oxidesat the interface between thereof and the gaseous and/or liquid medium.In this case, the Al₂ O₃ layer is deposited directly to the layer ofcerium oxides, while the SiO₂ layer, either to the Al₂ 0₃ layer or tothe layer of cerium oxides.

The refractive index of the film of cerium oxides is 2.1 to 2.3.Therefore, to prevent an additional light loss at the interface betweenthereof and the gaseous or liquid medium having a lower refractiveindex, 1.0 to 1.4, use should be made of a transition layer of one ortwo materials transparent to the erbium ion excitation light and havinga refractive index depending on the refractive index of the contactmedium. For example, if such a medium is air or a gas having arefractive index n=1, either a matching layer having a refractive index1<n<1.3 or a composition of two matching layers having successivelyreducing refractive indices within the above specified range should beintroduced to reduce the cerium oxides film - surrounding mediuminterface reflection light loss.

Since Al₂ O₃ and SiO₂, having refractive indices n=1.69 and n=1.46,respectively, have no intrinsic absorption bands in the Er3+ ionexcitation region, they fully satisfy, due to their thermo- andphoto-stability and chemical inertness, all requirements to materialsfor matching layers.

To reduce the light interference loss in Al₂ O₃ and/or SiO₂ layers, thelayer thickness should exceed the length of the ray path in thematerial, which corresponds to the second interference maximum, orshould be 1.5 times greater than the path length calculated from thefirst interference minimum for light with the maximum wavelength of theactive ion excitation spectrum or the pumping light source radiationspectrum, respectively. The maximum wavelength for Er³⁺ -dopedfour-level lasers is 0.6 μm, and for three-level lasers, about 1.5 μm,but since the radiation spectra of discharge lamps are limited by awavelength of about 1 μm, a wavelength of 1 μm is chosen for calculationin all cases. The path calculation should be performed for the normallight incidence at a minimum path length in the material sinceinterference exhibits itself stronger in thinner layers, and for skewrays it is considerably attenuated. The creation of conditions underwhich interference suppression is provided at a minimum transition layerthickness for all wavelengths and angles of incidence of light allows toprovide the transition layer efficiency regardless of the surroundingmedium refractive index and angles of light passage i n the reflector.An effectively "thick" layer in the most unfavorable case at the lowestsurrounding medium refractive index of 1 is a layer having a thicknessof 1.5 times greater than the path length calculated from the firstinterference minimum at a wavelength of 1 μm. For SiO₂ at a path lengthof 0.34 μm such is a layer of 0.51 μm in thickness. With allowance madefor accuracy and adaptability to manufacture the minimum thickness ofthe SiO₂ layer is determined to be 0.5 μm. For Al₂ O₃, an increase inthe optical path length per unit of layer thickness proportional to anincrease in the refractive index from 1.46 to 1.69 results in a decreasein the admissible minimum transition layer thickness to 0.3 μm, ascompared to the SiO₂ layer.

The upper limit of the thickness of the Al₂ O₃ and Si0₂ layers of 1.0 μmis determined by that a further increase in the thickness causes thelayer recrystallization by radiation, which results in an increase inthe light scattering loss and a decrease in the efficiency of lighttransfer from the pumping source to the active medium.

The set task is also achieved by using a liquid medium having filteringproperties and containing cerium compounds, which is interposed betweenthe pumping source and the active medium. This provides effectivecontinuous operation of the Er³ +-doped laser at a high average pumpingpower.

The combination of a filter and a liquid medium provides uniformdistribution of absorption centers over a thicker layer and allows ahigher concentration thereof as compared to solid solutions (quartz andother glasses), without causing stresses and damages resulting from thefilter heating by the absorbed UV light. Therefore, a liquid filterallows a greater specific pumping power as compared to glass orthin-film filters. The cerium absorption centers may be contained in theliquid base composition or in additives to it, due to which the requiredoptical density of the filter is readily obtained over the wavelengthrange below 320 nm, and the transmission of waves of above 360 nm isprovided by cleaning the medium from other ions. The liquid medium maybe very viscous, for immersion of the active medium and/or the pumpinglight source in uncooled lasers, or may have a minimum viscosity, incase it is used as a coolant for cooling the light source and/or theactive medium. The enhancement of the laser output energy and power isachieved by using the filtering liquid medium simultaneously as acoolant for cooling the high-power pumping light source and/or theactive medium at a high output radiation power and for effectivefiltering the short-wavelength pumping radiation. For this purpose usecan be made of cerium salts not containing oxygen and/or organic groupscausing decomposition of cerium compounds and the removal of cerium inthe form of precipitate or gels from the liquid medium, and, primarily,used as the liquid medium can be water solutions of cerium halide salts.

The use of a filtering liquid medium in the form of a water solution oftrivalent cerium halides for isolating the active medium from the lightsource and as a liquid coolant provides the narrowest transmissioncutoff just within the 320 to 360 nm spectral region. This is determinedby the smallest broadening of Ce3+ absorption lines in water-solvatedCe³⁺ salts, and the substitution of Ce.sup. 3+ for Ce⁴⁺ shifts theabsorption band into the short-wavelength region, in which case asufficient filter density over the 290 to 320 nm wavelength range, wherelight induces color centers based on iron ions, may be not provided.

The most effective is a solid-state laser comprising a liquid mediumbased on water solution of trivalent cerium chloride salts (CeCl₃) witha salt concentration of at least 0.4 M/l.

In some cases, when the presence of chlorine ions is intolerable forreason of corrosion (for example, when vinyl chloride plastics are usedin contact with a liquid medium containing chlorine ions), a practicableand sufficiently effective variant of the laser is a solid-state lasercomprising a liquid medium based on a water solution of trivalent ceriumbromide salts (CeBr₃) with a salt concentration of at least 0.4 M/l.

Since cold-resistant coolants for solid-state lasers may contain halidemixture solutions, used as a liquid medium in a laser may be a liquidmedium based on a water solution of trivalent cerium chloride salts,which contains trivalent cerium bromide salts with a total concentrationof cerium salts of at least 0.4 M/l.

A water-based solution used as a liquid medium in a laser allowsintroduction of other salts or organic additives required in eachparticular case without degradation of the required spectral properties,provided the water amount necessary for CeCl₃ and CeBr₃ is maintained.

It should be noted that the highest efficiency of the claimed inventionbecomes apparent in lasers using an Er³⁺ -doped active medium based onyttrium aluminate crystals (YAlO₃). In spite of hypersensitivity of theYAlO₃ :Er³⁺ crystals to UV radiation, this approach provides long-timeserviceability of such lasers without a loss in the lasing efficiency.To do it by using another method is a complicated thing, especially whenemploying xenon pumping lamp pulses with a high specific power and alarge UV component in the radiation spectrum.

Another effective variant is the use of an Er³⁺ -doped active mediumbased on yttrium-aluminum garnet (YAG) crystals (YAG:Er³⁺).

Effectively used with the above described filters can be any activemedium doped with ions the effective excitation wavelength range ofwhich has a short-wavelength cutoff of not below 350 nm, for examplewith Nd³⁺ ions (YAlO₃ :Nd³⁺ or YAG:Nd³⁺).

FIGS. 1 and 2 illustrate a schematic and cross sectional elevation of apreferred embodiment of the present invention. A cylindrical pumpinglight source 1 is enveloped by a layer of transparent material 2. Afilter film 3 of cerium oxides is deposited on the surface of thetransparent material. The cerium oxides film 3 is generally 0.3-1.5 μmthick and has a stoichiometric composition comprising at leasttwo-thirds CeO₂. A transition layer 4 in the form of a film of aluminumtrioxide (Al₂ O₃) having a thickness of 0.3 to 1.0 μm is deposited onthe surface of the cerium oxides filter film. An additional transitionlayer 5 of silicon dioxide (SiO₂) is deposited on the surface of thealuminum trioxide. A liquid medium 6 having filtering properties andcontaining cerium compounds is interposed between the coated pumpingsource and the active medium 7 which is based on either of erbium-dopedyttrium aluminate crystals or yttrium-aluminum garnet crystals. Theliquid medium 6 is generally a water-based solution of cerium chloride(CeCl₃) or cerium bromide (CeBr₃) or a combination of both and whereinthe total concentration of cerium salts is not less than 0.4 M/l.

VARIANTS OF THE EMBODIMENT OF THE INVENTION

When a laser design comprising a medium doped with Er³⁺ ions isemployed, a total thickness of filtering layers of cerium oxides of 1.0to 1.5 μm is sufficient for removing the pumping light UV component,whereas the scattering loss in a thinner layer decreases due to theimproved film quality. Therefore, the use of a filter in the form of twoor three layers enhances the lasing efficiency.

In some cases it is advisable to use a coating of 0.5 to 0.8 μm inthickness deposited to two different surfaces of transparent media, forexample, the pumping source envelope and the active medium, or to one ortwo walls of optical components between thereof.

The presence of if only a single optical component between the lightsource and the active medium increases the number of surfaces fordepositing up to three or four coatings, and the thickness of the layerof cerium oxides in such cases can be reduced to 0.3 to 0.4 μm. Thisenhances the coating optical quality, strength and durability due to afurther reduction of the thickness and improvement of the structure ofthe layer in the course of deposition and annealing at the filter totalthickness of 1.2 to 1.6 μm. In this case, a coating of a greater totalthickness (up to 6 μm) for use with the most UV-sensitive activeelements of yttrium aluminate with a high erbium ion concentration canbe readily realized, if required, by increasing the thickness of eachfilm of cerium oxides up to 1.5 μm.

Thin layers are also applicable in case the pumping light sourceenvelope is made of a filtering material containing cerium oxides (CeO₂and Ce₂ O₃). At a low admissible concentration of CeO₂ and/or Ce₂ O₃doped into quartz glass the latter has an enhanced transmittance in thewavelength region below 300 nm. Therefore, to increase the total filterdensity required to satisfy the spectral requirements to the laser, itis sufficient to use an additional layer not thicker than 1 μm,depending on the thickness and properties of the envelope glass.

To reduce the light interface loss, the most optimum when the contactmedium is a gas or water is a coating comprising a 0.3 to 1.0 μm thicklayer of Al₂ O₃ having a refractive index n=1.69 and deposited directlyto the layer of cerium oxides having a refractive index of 2.1 to 2.3and a 0.5 to 1.0 μm thick layer of SiO₂ having n=1.46 and deposited, inits turn, to the Al₂ O₃ layer. In this case, a noninterference coatinghaving a stepwise reducing refractive index: namely, 2.3, 1.69, 1.46,1.33-1.0, is formed which introduces the lowest light loss in the erbiumion excitation range.

In case the laser comprises a liquid medium having a refractive indexn=1.4 to 1.5, it is sufficient to use a single transition layer of Al₂O₃ film having n=1.69 since a SiO₂ layer does not give a furtherreduction of loss and only impairs the filtering layer cooling. In casesthe surrounding medium refractive index is 1.0 to 1.3 and matching ofthe thermal coefficient of expansion of the transition layer with thatof the transparent medium material under the filtering film is required,it is advisable to use only a single SiO₂ layer deposited directly tothe coating of cerium oxides. Such a variant can be effectivelyemployed, for example, when a coating is deposited to the light sourceenvelope quartz glass.

A concrete example of a laser design with a filter in the form of acoating is a solid-state laser based on erbium-doped yttrium aluminateand comprising a CeO₂ filter coating deposited to the pumping lamp and aSiO₂ transition layer. To obtain an optimum pumping light filtering, thethickness of the CeO₂ coating on the lamp envelope for operation withthe erbium-doped active medium should be 1.0 to 1.5 μm with one of abovedescribed loss-reducing matching coatings consisting of a film of SiO₂of 0.5 to 1.0 μm in thickness. In this case, the filter optical densityin the λ<320 nm region exceeds 3, and at λ>360 nm is 0.08 to 0.1 with asharp reduction to below 0.03 at λ=370 nm. An example of a variant of alaser design comprising a liquid filtering medium is a laser in whichthe active medium and the pumping light source are cooled with CeCl₃water solution with a salt concentration of 4 to 6 M/l at a totalthickness of the solution layer in the coolant channels around theactive medium and/or the pumping light source of 3 to 1 mm. In thiscase, the filter transmission spectrum cutoff width is 30 nm, thedensity at λ=360 nm does not exceed 0.04 to 0.05, and in the spectralregion below 320 nm, exceeds 3.

An experimental laser using φ5*50 mm YAlO₃ :Er³⁺ elements and a xenonpumping lamp with a φ3*45 mm discharge gap has been developed, in whoseflat resonator with the output mirror transmission of 10 to 20% athreshold of 6 to 7 J at a differential efficiency of about 0.2% hasbeen obtained. In this case, at a pumping energy of about 17 J an outputenergy exceeding 20 mJ at pulse repetition rates of 10 to 50 Hz has beenobtained. In the world literature there have been no reports ofintermittent lasing at such a threshold pumping energy, differentialefficiency and pulse repetition rates at λ=1.66 μm simultaneously.

Similar high results have been obtained in lasers with an active mediumin the form of several active elements and a pumping light source in theform of several straight discharge lamps at various combinations of thenumbers of active elements and lamps. For example, to relieve load onthe lamps at the operating active medium pumping exceeding the energy orpower allowable per one lamp, the use was made of a laser configurationcomprising two lamps and a single active element at the filteringvariant with the use of water solution of CeCl₃ salt in the coolingjackets of both lamps and of the active element simultaneously.

In all above cited examples of laser designs the useful life of theactive elements has increased by at least 100 fold, which fact allowedthe laser continuous operation without a reduction of its outputcharacteristics and without an increase in the lasing threshold at pulserepetition rates of up to 100 Hz.

Thus various variants of skin- and eye-safe lasers having a lowthreshold pumping energy and high efficiency and lasing pulse repetitionrate and emitting at a wavelength of 1.6 to 1.8 μm have been devised.

Since the designs offered above are effective for lasing at a wavelengthof 1.6 to 1.8 μm from the ⁴ S_(3/2) level within the Er³⁺ ion levelscheme above the ⁴ I_(11/2) level from which lasing at a wavelength of2.7 to 3.0 μm takes place, they can be effectively used in lasers basedon Er³⁺ -doped media at a lasing wavelength of 2.7 to 3.0 μm.

INDUSTRIAL APPLICABILITY

The present invention is applicable in sea and air transport, geodesyand cartography, signaling systems, environment monitoring, and inmedicine.

The most successfully the invention can be embodied in the form ofmonopulse lasers emitting in the atmospheric transmittance region atwavelengths ranging between 1.5 and 1.8 μm and used for distancemeasurement and optical detection and ranging. The lasers emitting atwavelengths of between 2.7 and 3.0 μm are effective for application inmedicine, including surgery.

We claim:
 1. A solid-state laser comprisingan active medium doped withtrivalent erbium ions; a pumping light source with an envelope of atransparent material isolated from the active medium; an additionalfiltering layer comprising at least one of the group consisting ofoptical components, coatings deposited thereon, a liquid medium,isolating the transparent material; wherein the additional filteringlayer is transparent to light over an excitation range of the activemedium, and wherein the filtering layer over a full length of a ray pathfrom the pumping source to the active medium, including a path in theenvelope, is characterized by an optical density of at least 2, forwavelength ranges below 320 nm, and an optical density of not more than0.1, for the active medium excitation spectral bands of wavelengthsabove 360 nm.
 2. The solid-state laser of claim 1, further comprising afilter consisting of a thick, hard, durable film of cerium oxides ofthickness between 0.3 and 1.5 μm, wherein the film has a stoichiometriccomposition having at least two-thirds tetravalent cerium dioxide(CeO₂), and wherein the film is deposited directly on a surface of atleast one of the envelope of transparent material, the optical componentand the active medium.
 3. The solid-state laser of claim 2, furthercomprising a transition layer deposited on the surface of the film ofcerium oxides, wherein the transition layer comprises aluminum trioxide(Al₂ O₃), the transition layer having a thickness in the range 0.3-1.0μm, and wherein the transition layer forms an interface between the filmof cerium oxides and a gaseous or liquid medium.
 4. The solid-statelaser of claim 2, further comprising a layer of silicon dioxide (SiO₂),having a thickness in the range 0.5-1.0 μm, deposited on the surface ofthe film of cerium oxides, and wherein the layer of silicon dioxideforms an interface between the film of cerium oxides and a gaseous orliquid medium.
 5. The solid-state laser of claim 3, further comprising alayer of silicon dioxide (SiO₂), having a thickness in the range 0.5-1.0μm, deposited on the surface of the transition layer of aluminumtrioxide, and wherein the layer of silicon dioxide forms an interfacebetween the transition layer and a gaseous or liquid medium.
 6. Thesolid-state laser of claim 1, wherein the liquid medium comprises ceriumion compounds.
 7. The solid-state laser of claim 6, wherein the liquidmedium comprises a solution of trivalent cerium chloride (CeCl₃) inwater, the solution having a salt concentration of at least 0.4M/l. 8.The solid-state laser of claim 6, wherein the liquid medium comprises awater solution of trivalent cerium bromide (CeBr₃) in water, thesolution having a salt concentration of at least 0.4 M/l.
 9. Thesolid-state laser of claim 6, wherein the liquid medium comprises asolution of trivalent cerium bromide and trivalent cerium chloride inwater, the solution having a total concentration of cerium salts of atleast 0.4 M/l.
 10. The solid-state laser of claim 1, wherein the activemedium doped with trivalent erbium ions comprises yttrium aluminatecrystals (YAlO₃ :Er³⁺).
 11. The solid-state laser of claim 1, whereinthe active medium doped with trivalent erbium ions comprisesyttrium-aluminum garnet crystals (YAG:Er³⁺).
 12. A solid-state lasercomprisingan active medium doped with ions having an effectiveexcitation wavelength range having a short-wavelength cutoff of at least350 nm; a pumping light source with an envelope of a transparentmaterial isolated from the active medium; an additional filtering layercomprising at least one of the group consisting of optical components,coatings deposited thereon, a liquid medium, isolating the transparentmaterial; wherein the additional filtering layer is transparent to lightover the excitation range of the active medium; and wherein thefiltering layer over a full length of any ray path from the pumpingsource to the active medium is characterized by an optical density of atleast 2 for wavelength ranges below 320 nm and an optical density of notmore than 0.1 for the active medium excitation spectral bands ofwavelengths above 360 nm.